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Published in "Coordination Chemistry Reviews 252(8-9): 856-885, 2008"
which should be cited to refer to this work.
Coordination polymer networks with s-block metal ions
Katharina M. Fromm
University of Fribourg, Department of Chemistry, Chemin du Musée 9, CH-1700 Fribourg, Switzerland
Contents
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1.
2.
3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Group 1 metal compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1. Neutral oxygen donor ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2. Anionic oxygen donor ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1. Alkoxides and aryloxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2. Carboxylates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.3. Sulfonates and nitro-derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.4. Amino acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.5. Mixed O- and N-donors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3. N-donor ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4. Carbon donor ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5. Sulfur donor ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Group 2 metal compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1. Neutral oxygen donor ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2. Anionic oxygen donor ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1. Beta-diketonates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.2. Alkoxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.3. Carboxylates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.4. Phosphonates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.5. Sulfonates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3. Mixed N- and O-donors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4. N-donor ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5. Carbon donor ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.6. Sulfur donor ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1. Alkali metal coordination polymer networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2. Alkaline earth metal coordination polymer networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Abstract
Alkali and alkaline earth metal cations form the s-block elements of the periodic table where they belong to the groups 1 and 2, respectively.
They play an important role in nature, for instance alkali cations Li+ , Na+ and K+ have very specific functions such as the regulation of the ionic
equilibrium of living cells in our body. Alkaline earth cations also have a contribution in our body, for instance calcium is the most important
constituent of our organism. They above all, and always, find applications in man-made materials in a wide range of fields: catalysts, ferroelectrics,
metallic conductors and superconductor materials.
They are known for their mainly ionic chemistry in aqueous medium, and a varying coordination number, depending on the size of the binding
partners as well as on electrostatic interactions between the ligands and the metal ions. This makes the strategic synthesis of coordination polymer
E-mail address: [email protected].
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networks with these metal ions a challenge and explains why few systematic results in the generation of metal–organic frameworks (MOFs) are
found in the literature. This review highlights the recent results in the field, bringing together the systematic approaches with results obtained by
serendipity, to give an overview on current and future possibilities.
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Keywords: Alkali metal ions; Alkaline earth metal ions; Coordination polymer networks; Metal–organic frameworks; s-Block metal coordination compounds
1. Introduction
2. Group 1 metal compounds
The coordination chemistry of groups 1 and 2 metal compounds with organic ligands in the widest sense has been,
until relatively recently, largely unknown compared to transition metal coordination networks. This is true despite the fact
that many s-block metal–organic compounds are already of commercial importance. Thus, pharmaceuticals, dyes, and pigments
typically use alkali and/or alkaline earth metal cations in preference to transition or lanthanide metal ions because most of
them have the advantage of being non-toxic, cheap and soluble
in aqueous media. Indeed, s-block cations should not be ignored
as simple “spectator” ions when it comes to properties which
depend on the solid-state structure and the intermolecular interactions. This is especially relevant in pharmaceutical industry
where one salt might be preferred over others for practical as well
as commercial purposes. Therefore, understanding the changes
of material properties caused by changing the s-block metal ion
is based on consideration of the fundamental properties such
as charge, size, and electronegativity of these cations and their
influence on the nature of the resultant solid-state structure. Furthermore, the chemistry of group 1/2 metal ions is not limited
to the classical ionic behavior as known from aqueous media,
but may exhibit a more covalent character similar to transition
metal compounds when polar organic solvents are used.
With the aid of modern X-ray diffraction techniques, a variety of molecular and polymeric structures can be elucidated.
Coordination polymer networks are made mainly from neutral or
anionic ligands (linkers) with at least two donor sites which coordinate to metal ions or aggregates (nodes) also with at least two
acceptor sites, so that at least a one-dimensional arrangement
is possible. Depending on the number of donor atoms and their
orientation in the linker, and on the coordination number of the
node, different one (1D)-, two (2D)- and three (3D)-dimensional
constructs are accessible. While systematic studies to construct
transition metal–organic frameworks (MOFs) are frequent in
the current literature, due to the often well-defined coordination
numbers of the metal ions, they are still sparse for s-block elements. Due to the span of coordination numbers possible for
alkali and alkaline earth metal ions, control over the final structure is a challenge, and many results are obtained by serendipity.
In this review, the current literature is highlighted, classified for
the group 1 metal ions, then group 2 metal compounds, as well
as for donor atoms in the organic ligands, mainly focusing on
the classical O-, N, and S-donors, but also including aromatic
rings as donor ligands. Thus, a first chapter will be dedicated to
alkali metal compounds with polymeric structures, starting with
organic O-donor ligands, followed by examples with N-donor,
C-donor, and S-donor ligands. The same systematic will be used
for group 2 metal ions.
2.1. Neutral oxygen donor ligands
Ethers in general (THF, Et2 O, etc.) are among the most frequently used solvents for alkali metal compounds. They are
usually polar enough to dissolve group 1 compounds with ionic
character because they are able to coordinate to the metal ions.
By acting as “chemical scissors” on alkali metal salts, they are
able to stabilize structural fragments of these compounds by
acting as neutral donor ligands where coordination is needed.
Such solvent adducts are frequent at the molecular level, but not
always identified when of polymer character due to the poorer
solubility of the latter. One example is LiBH4 , forming different one-dimensional ether adducts which can be isolated and
characterized as 1D structures in the solid state (Fig. 1), i.e.
LiBH4 (OEt2 ) and LiBH4 (OMetBu) [1].
Instead of monodentate ethers, cyclic polyethers can also be
used to stabilize low-dimensional structures of alkali metal salts.
Since the pioneering discovery of crown ethers and cryptands
by Pedersen, Cram and Lehn, coordination compounds based on
these ligands have been studied in considerable detail. Crown
ethers with four to six oxygen atoms are in principle predestined
for building one-dimensional channel structures. Much effort
has been made to obtain such structures for ion conductivity.
However, most syntheses lead to structures in which the alkali
metal ions are (i) coordinated by the crown ether and (ii) bridged
by water molecules to yield a one-dimensional (1D) zig-zag
structure without alignment of the crown ether molecules. Only
few examples are known from the literature, in which the crown
ether ligands are also perfectly stacked to form a channel system.
One of these is [Na ⊂ (DB18C6)(␮-OH)][Na ⊂ (DB18C6)(␮H2 O)]I3 (Fig. 2) [2]. These systems do show transport properties
for ions as well as water molecules [2], and could therefore be
interesting for solid-state batteries.
Another way of stabilizing one-dimensional structures is by
using crown ether molecules, which are too small to accommodate the cation in their centre. In this case, coordination of the
crown ether towards a metal ion would take place in a side-on
fashion, as exemplified in Cs(18C6)(N3 )(H2 O)(MeOH) (Fig. 3)
[3]. For the smaller alkali homologue compounds, molecular
mono- or dimeric entities are observed.
Going from molecular ligands such as these crown ethers
to 1D polyethers such as PEO (polyethylene oxide), polymer
electrolytes of group 1 metal ions are obtained, finding applications in ion rechargeable (lithium ion for instance) batteries.
Their exact solid-state structure is not well known, but some
studies on lithium, sodium and potassium reveal the coordination of the PEO ligand to the metal ions. Almost independently
from the anions, the polyether oxygen atoms wrap around the
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Fig. 1. Ether stabilized 1D structure of LiBH4 (OEt2 ); with permission from [1].
Fig. 2. 1D structure obtained within crown ether ligands [2].
alkali cations to form single or double helical structures with
the metal ions in the center of a channel-like feature [4]. The
mobility of lithium ions was studied by molecular dynamics
simulations, where experimental and calculated X-ray diffractograms are compared for a good fit [5]. Other polyelecctrolytes
derived from PEO, such as polyether poly(urethane)urea have
been studied in a similar context [6].
Derived crown ether ligands such as the dithia[16.3.3](1,2,6)cyclophane also yield coordination polymer structures, such as
a 1D zig-zag chain of KClO4 stabilized by side-on coordination
of the cyclophanes (Fig. 4) [7], whereas the sodium analogue
turns out to be a 0D dimer.
Certain functionally (phenolic) substituted azacrown ethers
such as N,N -bis(4-hydroxy-3,5-dimethylbenzyl)-1,4,10,13-
Fig. 3. 1D structure obtained by side-on coordination of crown ether ligands; with permission from [3].
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Fig. 4. 1D coordination polymer obtained by the coordination of dithia[16.3.3](1,2,6)cyclophane to KClO4 ; adapted from [7].
tetraoxa-7,16-diazacyclooctadecane can act as bridging ligands
to build polymer compounds in which they play the role of
linker, whereas so far we have seen the ether ligands as coordinating, but not as bridging entity [8]. Sodium and potassium
thiocyanate complexes are formed, in which the phenolic OH
group in the side arm of the crown ether coordinates to the
cation incorporated in the crown moiety of another molecule
(Scheme 1).
Fourier-transform IR spectra of such compounds show
that the O–H stretching band of the phenolic OH group
in the polymer-like complexes shifts to lower frequency by
ca. 210–290 cm−1 compared with those of the corresponding host compounds. 13 C NMR titration experiments indicate
that these polymeric structures are however not maintained in
solution.
An emerging approach in the rational assembly of networks
is through the use of secondary building units (SBUs), which
are most commonly metal-containing aggregates that dictate the
direction of polymer extension. The use of SBUs is attractive
in part because of their steric requirements and rigidity, which
dramatically reduce the number of possible network topologies arising for a given node/linker combination. Application
of the SBU concept has mainly focused on inorganic solid-state
chemistry and also on covalently linked transition metal cage
complexes.
If this construction method is used together with a template anionic building block such as a carbaborane, the
neutral oxygen donor ligand cyclotriveratrylene (CTV) may
build low-dimensional structures, for instance connecting
[Na2 (H2 O)2 (dmf)2 (␮-dmf)2 ]2+ -clusters into a distorted hexagonal coordination polymer (Fig. 5)[9].
Using the simpler 1,4-dioxane as polyether ligand, it can act
as terminal and/or connecting unit between metal ion clusters,
leading to different dimensional polymer structures, as exemplified by the 1D-chains, 2D-sheets and 3D diamondoid network
of, respectively, [{(ROLi)4 . (dioxane)x }], R = Ph, x = 3; R = 4Et-C6 H4 , x = 2.5; and R = 1-naphth, x = 2 (Fig. 6) [10a]. Other
tetrahedral nodes to generate a diamondoid network with 1,4dioxane are based on the smaller Li2 O2 -ring dimers formed from
(4-Br-2,6-Me2 C6 H2 OLi) [10b].
This example of the literature is reminiscent of transition
metal coordination polymer structures and leads over to the
following compounds in which anionic oxygen donor ligands
play important roles in the formation of coordination polymer networks. However, if the supposedly bridging bidentate
donor ligand is not “strong” enough to displace anions, respectively to coordinate in the expected bridging fashion, or has
the wrong orientation, molecular entities are obtained, such as
in [Na ⊂ (DB18C6)I(1,3-dioxolane)], where 1,3-dioxolane is a
potential bridging ligand, but with a poor orientation and interfering with the crown ether ligand [11]. If such steric factor is
eliminated, 1,3-dioxolane can act as ligand on tetrahedral nodes
to yield a diamondoid structure [10b].
Scheme 1. Scorpionate-type crown ether ligands for the construction of 1D
alkali metal polymer chains; by permission of the Royal Society of Chemistry
from [8].
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Fig. 5. Structural details of [Na2 (dmf)4 (H2 O)2 (ctv)][(dmf)0.5 (ctv)][CB11 H6 Br6 ]2 , with permission from [9].
2.2. Anionic oxygen donor ligands
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So far, we have looked at inorganic salts or clusters of alkali
metal ions to which neutral oxygen donor ligands are coordinated, and which possess 1D, 2D or 3D structures. This chapter
will now deal with organic, anionic ligands with a negative
charge on the oxygen atom. The first examples to cite here are
alkoxides, aryloxides and carboxylates.
2.2.1. Alkoxides and aryloxides
One of the best known polymers of alkali alk- and aryloxides, MOR, are of course the methoxides and simple phenoxides
which form insoluble compounds based on chain or sheet structures [12–18]. Structures can be tuned as a function of the steric
demand of R, and thus, the larger R, the more aggregates (0dimensional) are obtained [19].
Aryloxides can form coordination polymer compounds as
well, and, additionally to the metal–oxygen interaction, the aromatic group can participate in the coordination to the metal ion.
This is especially the case for the heavier and thus softer alkali
metals, such as Rb or Cs (Fig. 7) [20].
Alkali metal salts of organic acids are numerous, and very
often not well characterized in the solid state. The presence of
two oxygen atoms in the acid function leads to a stronger bridging function than for alkoxides, resulting more typically in the
formation of layered structures [21–24].
The use of cyclic polyphenols, like calixarenes, allows isolating one-dimensional coordination polymer compounds in
which the cations can even be mobile similar to biological
ion channels. One such example is given in [K(calix[8]areneH)(THF)4 (H2 O)7 ] (Fig. 8) [25].
2.2.2. Carboxylates
One other interesting example is the synthesis of ferromagnetic 2D-coordination polymer compounds from nitroxyl
free-radical carboxylic acids (4-carboxy-TEMPO and 3carboxy-PROXYL, see Scheme 2) and sodium or potassium ions
(Fig. 9) [26].
Fig. 6. Different dimensional polymeric structures depending on the number of
bridging dioxane ligands; by permission of the Royal Society of Chemistry from
[10].
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Fig. 7. 1D structure of [Cs(2,4,6-tBu3 C6 H2 O)(DME)]; with permission from [20].
Fig. 8. Part of the 1D structure of [K(calix[8]arene-H)(THF)4 (H2 O)7 ] [25].
Combining the anionic carboxylate groups with neutral N-donor sites within one ligand, such as the 3,5pyridinedicarboxylate (3,5-pdc), the dimensionality of such
polymers can be increased into a 3D-network, as exemplified
in [Na2 (3,5-pdc)(H2 O)4 ], in which additional H-bonding occurs
[27].
Upon addition of a further coordinating anion, the layers
of Na-acetate for instance can be cross-linked into a threedimensional porous network with channels in which the solvent
molecules can be found (Fig. 10) [28].
2.2.3. Sulfonates and nitro-derivatives
Apart from aryl-, alkoxides and acid derivatives, sulfonates
are an important class of oxygen donor ligands to alkali metal
ions. A ligand combining sulfonate groups and aryloxide groups
is Orange G, a synthetic azo dye used in histology in many staining formulations. One difficulty in examining sulfonated dyes is
that they exhibit generally poor crystal growth properties, mak-
Scheme 2. 4-Carboxy-TEMPO (left) and 3-carboxy-PROXYL (right); by permission of the Royal Society of Chemistry from [26].
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Fig. 9. Ferromagnetic 2D coordination polymer of 4-carboxy-TEMPO with Na; by permission of the Royal Society of Chemistry from [26].
are rationalized in terms of simple properties of the metals
(charge, size, and electronegativity) and the steric demands of
the arylsulfonate groups. This is summarised by saying that the
most electronegative metals favor M–OH2 bond formation and
hence solvent-separated ion-pair structures, whereas the most
electropositive metals form higher-connectivity complexes with
more M–O3 S bond formation. Thus, the sodium derivative forms
a two-dimensional extended structure, involving the sulfonate
groups as well as the keto O-atom in the coordination of the
metal ions as well as bridging water molecules (Fig. 11).
Another application of sulfonates in combination with other
donor groups is found in high-energy materials. It is well
known that the nitration products of phloroglucinol (PG)
ing single-crystal diffraction studies difficult. An exception is
the disulfonated naphthalene-based dye Orange G, 7-hydroxy-8(phenylazo)-1,3-naphthalenedisulfonic acid, the salts of which
are found to grow as robust crystals [29]. The s-block metal
salts of the Orange G dianion (Og) can be categorized into three
structural classes related to those previously proposed for simple
monosulfonated azo dyes. All of the structures feature alternate organic/inorganic layering, but whereas the Mg, Ca, and Li
complexes are solvent-separated ion-pair species, the Sr and Ba
complexes form simple discrete molecules based on metal sulfonate binding bonding, and the heavy alkali metal complexes
utilize a variety of MO interactions to form two- and threedimensional coordination networks. These structural differences
Fig. 10. The Na4 (OAc)3 (ClO4 )·0.25(MeOH) structure viewed down the c axis; with permission from [28].
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Fig. 11. Structure of the dianion of Orange G (left), and structure of the Na-derivative (right); with permission from [29].
Fig. 12. Structure of DNPGSH (left) and 3D structure of [K(DNPGS)(H2 O)]; with permission of Springer from [30].
are important intermediates in chemical industry and can
be used as such high-energetic materials. Additionally, in
recent years, potassium energetic coordination compounds have
received extensive attention due to their potential properties
such as low toxicity and minimal pollution. A novel coordination polymer, potassium 3,5-dinitrophloroglucinolsulfonate
monohydrate [K(DNPGS)(H2 O)], was synthesized by the reaction of 3,5-dinitrophloroglucinolsulfonic acid (DNPGS) with
potassium hydroxide in aqueous solution, yielding a threedimensional (3D) network with the participation of the sulfonate,
the phenolic and the nitro groups of the ligand, as well as coordinating, bridging water molecules (Fig. 12) [30].
Along these lines, the cesium derivative of 3,5-dihydroxy2,4,6-trinitrophenolate is relevant [31]. In this compound, each
Cs is coordinated to 12 oxygen atoms from eight different 3,5dihydroxy-2,4,6-trinitrophenolate (DHTNP) anions, and each
DHTNP-anion is coordinated with eight Cs cations. The oxygen atoms of the phenolic hydroxyl group and from all of the
nitro groups of the DHTNP-anion simultaneously bind to neighboring Cs ions. These DHTNP molecular chains are interlaced,
with the cesium atoms acting as knots, to create an infinite threedimensional network structure. The molecules are linked by
electrostatic interactions and weak van der Waals forces and also
display intramolecular hydrogen bonding, which all enhance
the thermal stability of this compound, which undergoes two
highly exothermic decomposition reaction processes from 166
to 300 ◦ C under the condition of a linear heating rate. The experimental sensitivity results indicate that [Cs(DHTNP)] is quite
sensitive to impact and friction (Fig. 13). Its properties resemble those of other trinitrophloroglucinol salts of alkali metals,
which can be used as components of ecologically clean initiating
compositions (Scheme 3).
Scheme 3. Proposed thermal decomposition of [Cs(DHTNP)] [31].
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Fig. 13. The coordination of Cs (left) and the packing arrangement of [Cs(DHTNP)]; adapted from [31].
2.2.4. Amino acids
Surprisingly little is known about the structures of
alkali metal derivatives of amino acids, which would
combine the acid donor group with the amine function.
Derivatives, such as the total-kill herbicide glyphosate [N(phosphonomethyl)glycine = H3 L], possess low mammalian
toxicity, are relatively quickly degraded in the soil and are
usually dealt with in form of alkali metal salts. For the
sodium salts, three different solvates of the monosodium compounds have been isolated, namely Na(H2 L)·0.5H2 O, sodium
glyphosate monohydrate Na(H2 L)·H2 O, sodium glyphosate
dihydrate Na(H2 L)·2H2 O, together with a disodium glyphosate
nonahydrate Na2 (HL)·9H2 O [32]. The first two compounds are
both polymeric and based on dimeric repeating units with six
coordinate sodium ions, while the disodium salt is based on both
discrete octahedral [Na(HL)(H2 O)5 ]− anionic complex units
with the unidentate glyphosate ligand bonded via a carboxylate
oxygen, together with a cationic [Na2 (H2 O)8 ]-ribbon polymer
(Scheme 4).
Even though this glycine derivate possesses three potential
donor groups for alkali metal ions, i.e. the phosphonate, the
carboxylate and the NH-group, the latter does not coordinate
to the metal ions in any of the observed structures. This is also
observed in the coordination mode of uracil-5-carboxylic acid
toward alkali metal ions Na–Cs, for which mainly 1D structures
(chains, tapes) are obtained [33].
In sulfonylamines, the donor groups, O- and N-atoms, are
now both charged negatively, and participate in metal ion coordination. The donor groups are relatively close together within
one ligand molecule, and thus, bridging coordination is preferred
over chelating action. This leads to the linear coordination polymer which consists of discrete square tubes based upon Na–O
and Na–N interactions (Fig. 14) [34].
For the larger cations potassium and rubidium of the
alkali metal series, the same ligand is used, but with an
increased coordination number of seven to yield water-free
three-dimensional compounds with a more regular structure
than the sodium compound (Fig. 15) [35]. In contrast, the
Cs-compound features anion monolayers that intercalate planar zig-zag chains of cations (Cs· · ·Cs alternatingly 422.5 and
487.5 pm, Cs· · ·Cs· · ·Cs 135.7◦ ), whereby each chain is surrounded and coordinated by four anion stacks and each anion
stack connects two cation chains. All structures exhibit close
C–H· · ·A interanion contacts consistent with weak hydrogen
bonding [35].
2.2.5. Mixed O- and N-donors
Polydentate ligands with anionic O-donors and neutral Ndonors can be obtained by integration of a cyano-function in
an appropriate position into anionic O-donors. Thus, 4-cyanosubstituted aryloxides, such as (4-NC-2,6-tBu2 -C6 H2 OLi), lead
to the formation of 1D chains via single metal ions stabi-
Scheme 4. Glyphosate and different derived polymer structures with Na-ions [32].
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Fig. 14. Chain interactions in Na4 [(MeSO2 )2 N]4 ·3 H2 O; with permission from [34].
lized by THF or pyridine ligands (Fig. 17) [36]. Instead of
using single metal ions as nodes, small cluster or aggregate
units can also be interconnected into chains, sheets or threedimensional motifs. As an example, lithium derivatives with
␣-cyanophosphonates, [(RO)2 P(O)CHCNLi] (R = Et, iPr), and
(organo)sulfonyl-acetonitriles, (RSO2 CHCNLi), as quasi linear bidentate ligands, tend to form 2D-layered structures by
linking Li2 O2 -units, respectively eight-membered ring dimers
(SO2 Li)2 , into sheets (Fig. 16) [37].
2.3. N-donor ligands
Given the fact that alkali metal ions are generally considered
as rather hard cations, they interact preferably with oxygen donor
atoms rather than nitrogen atoms. However, when oxygen donors
are not available in sufficient quantity to satisfy the coordination
of the metal ions, nitrogen atoms are also involved in metal ion
coordination. This is the case in the tripodal, pyridyl-substituted
tris(pyrazolyl)borate ligands [38]. Whereas the water-rich potassium derivative of this ligand type yields a dimeric structure, the
compound with only one water molecule in the potassium coordination sphere leads to a one-dimensional coordination polymer
network, in which two of the three arms of the tripodal ligand act as bridging ligands towards two neighbor metal ions
(Fig. 17).
Pure nitrogen donation is rare, as shown in the
example of the toluene-soluble potassium salt of the [(2,6iPr2 C6 H3 )NC(Me)C(H)C(Me)N(2,6− iPr2 C6 H3 )]-ion, in which
the aromatic rings complete the coordination sphere of the potassium ion by metal–aromatic interactions. This kind of interaction
is finally responsible for the 1D-extension of the structure in the
solid state (Fig. 18) [39].
Similarly, cyano-cyclopentadienyl (CpCN) anions are ideal
to combine nitrogen and aromatic ring interactions with
alkali metal ions, such as in CpCNM, M = K, Cs (Fig. 19)
[40].
Fig. 15. Isostructural structure for MN(SO2 Me)2 (M = K, Rb; left), and the Cs-compound (right), adapted from [35].
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Fig. 16. (a) Cyano-aryloxide as 1D-bridging ligand towards single Li-ions, adapted from [36]; (b) Assembly process of lithiated ␣-cyanophosphonates by linking
Li2 O2 dimers; adapted from [37].
This leads us to the coordination compounds of alkali metal
ions with carbon donors, of which phenyl groups and cyclopentadienyl (Cp)-ligands are the most representative in the formation
of extended structures.
2.4. Carbon donor ligands
The larger alkali metal ions are softer than the smaller
ones, and accept more easily aromatic coordination. This has
been shown in the zig-zag-arranged structures of the solidstate structures of CpM-compounds for the heavier alkali metal
ions [41,42]. Substituted phenyl rings with small and noncoordinating side-groups maintain such a chain-structure in the
solid state (Fig. 20) [43]. The derived calcium compound is a
THF adduct and by the way a monomer.
In combination with phosphor donor atoms which are linked
to a cyclopentadienyl-ligand and additional THF, a new 1D struc-
Fig. 17. Solid-state structure of the potassium tripod coordination polymer with
only one water molecule in the metal coordination sphere; adapted from [38].
ture can be stabilized, as for instance in the solid-state compound
[(THF)K(C5 H4 CH2 CH2 PPh2 )] (Fig. 21) [44].
Solvent adducts of CpM- and indenyl-M-compounds with
diethyl ether or polydentate amines lead either to molecular structures or also polymeric zig-zag chains, as in the
case of CpK(OEt2 ) (multidecker structure), [K(indenyl)(L)]
(L = TMEDA (N,N,N ,N -tetramethylethylenediamine) or
PMDTA (N,N,N ,N ,N -pentamethyldiethylenetriamine) [45].
Such different coordination modes are the combined result of
electrostatic and steric effects.
Single metal–carbon bonds with more σ-character can
be enforced with carbanions carrying large substituents like
Fig. 18. 1D structure of
iPr2 C6 H3 )]; adapted from [39].
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K[(2,6-iPr2 C6 H3 )NC(Me)C(H)C(Me)N(2,6-
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Scheme 5. Different coordination polymers obtained with Na and K; by permission of the Royal Society of Chemistry from [47].
2.5. Sulfur donor ligands
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Fig. 19. Zig-zag chains of cyanocyclopentadienyl alkali salts (K left, Cs right);
adapted from [40].
Fig. 20. Typical solid-state structure of aryl-potassium interactions; adapted
from [43].
phosphine and borane rests [46]. The related potassium
compound yields then a complicated sheet structure in
which additional K–H bonds seem to stabilize the structure
(Fig. 22).
Alkali metal coordination polymers with solely sulfur atoms
as donors, have not been reported in the literature so far.
However, as shown in some of the previous examples, the combination of different donor types allows the selective coordination
of different metal ions to different donor sites, as anticipated
from Pearson’s hard-soft acid–base theory. Thus, the Janus scorpionate ligand [HB(mtda)3 − ] (mtda = mercaptothiadiazolyl)
contains both sulfur and nitrogen atoms. This feature can
be exploited for the controlled construction of homo- and
hetero-metallic (hard-soft) alkali metal coordination polymers
(Scheme 5) [47]. Remarkably, in the case of sodium, coordination polymers with both acentric (with NaS3 N3 H kernels) and
centric (with alternating NaN6 and NaS6 H2 kernels) chains are
found in the same crystal (where the centricity is defined by
the relative orientations of the BH-bonds of the ligands along
the lattice) (Fig. 23). For the homometallic potassium congener,
the larger cation size, compared to sodium, induced significant distortions and favored a polar arrangement of ligands in
the resulting coordination polymer chain. An examination of
the solid-state structure of the mixed alkali metal salt system
revealed that synergistic binding of smaller sodium cations to
the nitrogen portion and of the larger potassium cations to the
sulfur portion of the ligand minimizes the ligand distortions relative to the homometallic coordination polymer counterparts,
a design feature of the ligand that likely assists in thermodynamically driving the self-assembly of the heterometallic
chains. The similarity of the solution analytical data regardless of the cation indicates that the salts are likely dissociated
in solution rather than maintaining their solid-state polymeric
structures.
Thiophosphonates combine the coordination of sulfur atoms
with the one of oxygen donors, within the same ligand. Thus,
the ligand [ArP(OtBu)S2 ]− (Ar = 4-anisyl) may coordinate with
both donor atoms to alkali metal ions in ethereal solvents to
yield mostly one-dimensional coordination polymer compounds
(Scheme 6 and Fig. 24) [48].
In terms of networks and nodes, moving from groups 1 to 2
metals halves the number of nodes and so decreases connectivity
for the same ligand types.
3. Group 2 metal compounds
Fig. 21. 1D structure of [(THF)K(C5 H4 CH2 CH2 PPh2 )]; by permission of the
Royal Society of Chemistry from [44].
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Passing from groups 1 to 2 metal ions, the charge density
of the cations increases. Also, many alkaline earth metal compounds are hygroscopic. For these reasons, many of the species
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Fig. 22. 2D structure of [(Me3 Si)2 {Me2 P(BH3 )}C]K; adapted from [46].
do contain water molecules, either directly linked to the metal
ions, or H-bonded to the formed structure in some way.
Until recently, the chemistry of alkaline earth metals has been
a largely underdeveloped area of coordination chemistry. There
have been a few studies on the coordination behavior of these
metals in both aqueous and non-aqueous media. The alkaline
Scheme 6. Reaction of thiophosphonates with alkali metal ions; adapted from
[48].
earth metals play an important role in the biochemistry of virtually all living organisms. They are distributed in most cells and
tissues, often in considerable concentrations, and a constant supply is indispensable for unrestricted performance of biological
functions. Magnesium and calcium are the most prominent alkaline earth metals in biological systems. Magnesium is present
solely as the solvated or complexed dication Mg2+ , basically
as a part of the osmotic equilibria in and outside of the cells
and in all tissues. Magnesium cations induce or stabilize tertiary
structures or enhance certain enzyme functionalities. Calcium
is the most prominent metal involved in structural biology. It is
the main constituent of bones and teeth. These two metal ions
play an essential role in the activation of enzymes, complexation with nucleic acids, nerve impulse transmission, muscle
contraction, and carbohydrate metabolism. A range of model
complexes containing these two metals have been prepared as
probes to elucidate the mode of binding of these metals. The
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Fig. 23. Polymeric 1D structures of M[HB(mtda)3 − ], (a) M = Na; (b) M = K; (c) M = Na/K; adapted from [47].
studies have focused on the use of aspartate, glutamate, orontate,
or pyroglutamate ligands as careful investigation of the calcium or magnesium binding sites on proteins has shown that
the acidic groups of l-aspartic acid and l-glutamic acid are the
key anchoring positions for these ions. On the other hand, barium and strontium are considered as trace elements in the body.
Strontium is rated nontoxic, and traces of it appear to be essential, though the reason for this is not yet known. Barium causes
typical heavy metal poisoning even at low concentrations if soluble compounds are applied. Barium and strontium metals have
been known as antagonists for potassium and calcium, respectively. In the process of binding to various sites, the ions compete
with other common metal ions, some of which are antagonists
through a different specificity. This specificity is predominantly
governed by the charge and size of the cation, as well as by
the resulting effective coordination number and geometry. An
improved understanding of the function of biological cations
is thus to be expected from a detailed study of the coordinating properties of the metals competing with various ligating
groups.
3.1. Neutral oxygen donor ligands
Similar to alkali metal ions, group 2 metal compounds can
be treated with for instance neutral oxygen donor ligands to
“cut out” fragments from the three-dimensional salt structure.
This has been exemplified in the studies on alkaline earth metal
iodides, which are still quite soluble in organic solvents which
may act also as coordinating ligands. In these cases, it is important to control the quantity of water present, as water is a solvent
Fig. 24. Section of the 1D structure of [K2 {ArP(OtBu)(␮-S)S}2 (dme)2 ]; by permission of the Royal Society of Chemistry from [48].
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Fig. 25. 1D structure of [BaI2 (thf)3 ] [55].
able to dissociate the mainly ionic bonds in alkaline earth metal
salts [49–54].
Thus, it has been shown that one-dimensional coordination
polymer compounds can be obtained with low concentrations
of THF, used as chemical scissors to yield [BaI2 (thf)3 ] (Fig. 25)
[55].
Similarly, by controlling the ratio of O-donor solvents, a
series of low-dimensional barium iodide solvates were isolated
[56]. This strategy could be transferred to alkaline earth triflate Ba(OTf)2 [57]. In these salt derivatives, the metal ions
are still directly coordinated by the anions and show excerpts
from their initial solid-state structure. Other neutral solvents are
however able to cleave the cation–anion contacts and to coordinate the cations completely, leaving the anions out of the first
coordination sphere of the cation. Water is such a solvent, and
thus, can replace one or both anions from the first coordination
sphere of the cation. As cation-coordinated water is a hydrogen bond donor ligand, low-dimensional polymer structures can
be designed as a function of the number of coordinating water
molecules [58]. As an example, the H-bonded one-dimensional
polymer [Ca(H2 O)2 (diglyme)2 ](␮-I)2 is cited [58–62].
Instead of using single metal ions as nodes to be connected
into 1D, 2D or 3D-motifs, cluster units can be used as well.
Thus, replacing terminal, monodentate O-donor cluster ligands such as THF by polydentate O-donors, such as DME
(dimethoxy ethane), the latter may displace the terminal ligands and bridge two cluster units into 1D chains of mixed metal
[SrLi6 (OPh)8 (thf)4 ]-clusters (Fig. 26) [63].
3.2. Anionic oxygen donor ligands
3.2.1. Beta-diketonates
Beta-diketonates of alkaline earth metal ions received prominent attention when High-Tc superconductors were discovered.
One synthesis of such oxide materials is via metal organic chemical vapor deposition (MOCVD) or the sol–gel technique. For the
Fig. 26. 1D structure of (␮-dme)[SrLi6 (OPh)8 (thf)4 ] (green: Sr, red: O, yellow:
Li and grey: C) [63].
first technique, volatile species are necessary. It turned out that
beta-alkoxides are well suited, because they form cluster-like
compounds in which the alkaline earth metal ions are surrounded
by the organic ligands and turn out to be volatile at relatively low
temperatures. From this property, the reader may deduce that
these compounds do not form coordination polymer network.
However, some cases are known, for which polymer structures
are obtained. Thus, beryllium, the smallest of the alkaline earth
metal ions, has a low coordination number of four, which in
the case of bis-beta-diketones leads to a one-dimensional chain
structure (Scheme 7) [64].
3.2.2. Alkoxides
Another group of alkaline earth metal compounds studied in
the same context are the alkoxides. Unless multifunctional alcohols’ and sterically crowded alkyl or aryl groups are employed
these compounds are generally polymeric insoluble solids,
which are poorly characterised from a structural point of view.
The reactions of these compounds with small molecules such as
CO2 and SO2 have been relatively neglected although organomagnesium compounds are known to react with SO2 to give
sulfinic acids after hydrolysis, while calcium oxide is used to
scrub flue gases and for the desulfurisation of iron ores. In
contrast, the corresponding reactions of the class ‘b’ or soft
metal alkoxides of palladium, platinum and rhodium with SO2
have been investigated in more detail. Structural studies of
these complexes reveal, as expected, S-coordinated SO2 moieties. In a communication by Arunasalam et al., the reactions
Scheme 7. 1D-coordination network of beryllium bis-beta-diketonates; adapted from [64].
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of [Mg(OMe)2 ], and [Ca(OMe)2 ]n with SO2 are described,
which lead to one-dimensional polymers, with good solubility
in organic solvents and which have interesting structural features in the solid state [65]. Thus, the reaction of SO2 (g) with a
suspension of [M(OMe)2 ], (M = Mg and Ca) in methanol yields
the crystalline compounds, [M(O2 SOMe)2 (MeOH)2 ], (M = Mg,
Ca). The insertion reaction is of high yield and the X-ray crystal
structure of the second complex reveals that it is a chain polymer with eight-coordinated Ca-ions and the methylsulfito anions
acting as both chelating and bridging ligands.
3.2.3. Carboxylates
Carboxylates were also among the species investigated as precursors for oxide materials, using sol–gel techniques. However,
many of these precursors may lead to the formation of mixed
alkaline earth metal carboxylates and oxides. This is why this
research was not further pursued. However, dicarboxylates have
found a large field of application in the construction of coordination polymer arrays, especially the more rigid ligands, as they
are useful building blocks for porous networks with metal ions
throughout the periodic system [66].
A semi-rigid ligand, benzene-1,4-dioxylacetate, L, leads
to the three-dimensional coordination polymer networks
[M(L)H2 O], M = Ca, Sr, Ba [67]. Both Ca(L)H2 O and Sr(L)H2 O
crystallize in the monoclinic space group P21 /c while Ba(L)H2 O
in the monoclinic space group P21 . As determined by X-ray
single-crystal analysis, in these compounds each metal ion is
coordinated by eight O atoms: four from different carboxylate
groups, two from one carboxylate group, one from the ether
oxygen and one from one water molecule (Fig. 27). Each ligand
coordinates to five alkaline earth metal ions through one of its
ether oxygen atoms and two carboxylate groups adopting novel
bridging coordination modes to give rise to a 3D network. The
luminescence analysis shows that the Ca- and Sr-compounds
exhibit fluorescence in the solid state at room temperature.
A claim for rational design based on metal ion coordination and H-bonding for the construction of polymer structures
with alkaline earth metal ions is the use of bicyclic bis-lactam-
dicarboxylate ions as bricks for two-dimensional compounds
[68]. The products are obtained by transmetallation of the alkali
salts by alkaline earth halides to yield different compounds
depending on the H-bonding capacity and the metal ion size
(Fig. 28).
The slightly more rigid zwitterionic 1,3-bis(carboxymethyl)imidazolium forms with SrCO3 in water a two-dimensional
structure between the layers of which are found sheets of water
molecules intercalated (Fig. 29) [69].
The very rigid 2,6-naphthalenedicarboxylate (ndc) reacts in
a solvothermal process with magnesium nitrate in either water
or DMF [70]. Whereas in the presence of water molecules preferred coordination of H2 O and dmf in [Mg(dmf)2 (H2 O)4 ]ndc
hinders the formation of extended three-dimensional networks,
a porous network structure is obtained in the absence of water
to yield the three-dimensional metal–organic framework of
[Mg3 (ndc)3 (dmf)4 ] (Fig. 30). The latter is microporous with
pores accessible to H2 , O2 , N2 and CH4 , and with a Langmuir surface area of 520 m2 g−1 and a hydrogen adsorption
capacity of 0.78 wt.% at 77 K and 760 Torr. The metal–organic
framework crystallizes in the space group C2/c [a = 13.451(3),
b = 18.043(4), c = 20.937(5) Å, β = 99.79(3)◦ ] with trinuclear
magnesium clusters connected to six dicarboxylate ligands that
link the clusters into a three-dimensional network. In contrast
to [Mg3 (ndc)3 (def)4 ] (def = N,N-diethylformamide), the dmf
molecules coordinated to magnesium in [Mg3 (ndc)3 (dmf)4 ] not
only cause a distortion of the network but induce accessibility
for the adsorption of nitrogen. So, only small structural changes
induced by coordinating dmf molecules play a key role in adjusting the pore size of the network and are responsible for the size
exclusion of nitrogen. Due to the low weight of magnesium,
the design of Mg-MOFs with higher specific surface areas is, in
principle, feasible, although identification of suitable structural
building units allowing for a high porosity remains a challenge,
for which the use of large rigid dicarboxylates and larger Mgclusters might be a solution.
A systematic study of larger flat aromatic ligands with two
carboxylic groups and beryllium and magnesium ions lead to a
Fig. 27. Coordination mode of the dicarbonyl ligand (left) and Ba-coordination polymer (right) of Ba(L)H2 O, L = benzene-1,4-dioxylacetate [67].
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Fig. 28. 2D structure of [M(C8 H8 O6 N2 )2 (H2 O)4 ], M = Ca, Sr; adapted from [68].
Fig. 29. 1,3-Bis(carboxymethyl)-imidazolium as O-donor ligand to Sr-ions in a layered structure [69].
large number of different metal organic frameworks with good
hydrogen absorption properties [71].
Increasing the number of carboxylate functions to three
in the flexible 1,3,5-triacetic acid (bta), layered structures of
[Ca3 (bta)2 (H2 O)8 ]·3H2 O and [Ba3 (bta)2 (H2 O)8 ] are obtained
(Fig. 31) .[72] Each bta3− ligand coordinates to Ca2+ or Ba2+
ions using two of three carboxylate groups adopting different coordination modes. The noncoordinated carboxylic group
forms hydrogen bonds with an adjacent network to generate
a three-dimensional structure. The complexes showed photo-
Fig. 30. 2,6-Naphthalenedicarboxylate as rigid ligand in the construction of porous and yet flexible metal–organic frameworks [70].
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Fig. 31. 2D structures obtained with 1,3,5-triacetic acid (bta); left: coordination to the calcium ions in [Ca3 (bta)2 (H2 O)8 ]3H2 O; right: details and schematic
representation of [Ba3 (bta)2 (H2 O)8 ]; adapted from [72].
luminescence in the solid state at room temperature. Probably,
water-free compounds with this ligand type would lead to higher
dimensional structures.
Finally, ligands with four carboxylic functions, such as
naphthalene-1,4,5,8-tetracarboxylate-hydrates (NTC), react
with alkaline earth metal chlorides to yield [Ca(NTC)(H2 O)2 ]·
H2 O, [Sr(NTC)(H2 O)2 ]·2H2 O, [Sr2 (NTC)2 (H2 O)8 ]·H2 O, and
[Ba(NTC)(H2 O)2 ] (Fig. 32) [73]. The NTC in the first two
compounds is tridentate, leading to two-dimensional layers,
and in the fourth pentadentate leading to a three-dimensional
network, forming bonds to the cations using the oxygen atoms
of the carboxylate and one oxygen atom of anhydride groups.
In the third compound the cations are associated only via the
oxygen atoms of carboxylate groups to form a chain structure.
Among the alkoxides, the carbonate anion can also be
included. As many gastropods and bivalves use calcium carbonate to build their shells, and/or to form nacre, this mineral
has been under investigation for the better understanding
of how these animals can construct very robust structures
[74]. A guanidinium-templated new carbonate-based network
with sodalite topology could thus be realized with magnesium and calcium ions [75]. Among alkaline earth metal
carbonates, [Mg6 (CO3 )12 (CH6 N3 )8 ]Na3 [N(CH3 )4 ]·xH2 O and
[Ca6 (CO3 )12 (CH6 N3 )8 ]K3 [N(CH3 )4 ]·3H2 O were isolated. In
these structures, pairs of guanidinium cations are associated
with the hexagonal windows of the sodalite cages, and alkali
metal cations are associated with their square windows. While
the Mg-compound is derived from a face-centered structure,
the calcium compound has a body-centered basis. The calcium ions have four close carbonate oxygen donors and four
other more distant ones, while magnesium has an octahedral
environment consisting of two bidentate chelating carbonate
ligands and two cis monodentate carbonate ligands (Fig. 33)
[75].
3.2.4. Phosphonates
A very early example of polymeric phosphonates with alkaline earth metal ions was actually reported in 1962. Thus, the
smallest alkaline earth metal ion, beryllium, is used for the
generation of one-dimensional coordination polymer based on
diphenylphosphonate, [Be(OPPh2 O)2 ] (Scheme 8) [76].
Since then, the investigation of phosphonates has more
focused on the more closely to nature related phosphonates.
Also, polyphosphonic acids have attracted significant attention in recent years due to their utility in supramolecular
chemistry and crystal engineering [77]. Polyphosphonates are
Fig. 32. 1D (left) and layered structure (right) of [Sr2 (NTC)2 (H2 O)8 ]·H2 O and [Ca(NTC)(H2 O)2 ]·H2 O, respectively; adapted from [73].
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Fig. 33. Environments of alkaline earth metal ions and carbonate ligands in (a) [Ca6 (CO3 )12 (CH6 N3 )8 ]K3 [N(CH3 )4 ]·3H2 O, which is representative of the BC
structural class, and (b) [Mg6 (CO3 )12 (CH6 N3 )8 ]Na3 [N(CH3 )4 ]·2H2 O, which is representative of the FC structural class; adapted from [75].
also used extensively in an array of industrial applications
such as chemical water treatment, oilfield drilling, minerals
processing, corrosion control, and metal complexation and
sequestration. Metal phosphonates commonly form pillaredlayered inorganic–organic hybrid materials and microporous
solids. Their properties can be useful for intercalation, catalysis,
sorption, and ion exchange. One example is the aminotris(methylenephosphonic) acid (H6 AMP). At low pH, this
Scheme 8. 1D-phosphonate network of beryllium [76].
ligand loses two protons, with all phosphonate groups being
deprotonated while the amino function carries a proton. This ligand can react easily with the alkaline earth metal cations Mg, Ca,
Sr, and Ba. Mg[HN(CH2 PO3 H)3 (H2 O)3 ] forms zig-zag chains
in which the Mg-ions are bridged by two of the three phosphonate groups. The third phosphonate group is non-coordinated
and involved in hydrogen bonding. A three-dimensional (3D)
polymer is obtained with strontium, having the composition
[Sr(H4 AMP)]. The Sr-atoms are seven-coordinate, with five
monohapto and one chelating AMP ligands. In the 3D barium polymer, [Ba(H4 AMP)(H2 O)], the two crystallographically
independent Ba-ions are 9- and 10-coordinated by phosphonate
and H2 O oxygen atoms (Fig. 34). All bridging oxygen atoms
belong to phosphonate moieties that act as chelates for one Ba2+
and form 4-membered rings. The crystal structure of the barium
compound is characterized by the complete absence of hydrogen
bonds.
With the more rigid phosphates containing strategically
placed bulky ortho-amide groups (Scheme 9), one-dimensional
chain structures can be obtained just as well as zerodimensional cyclic complexes. This has been shown by
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Fig. 34. Coordination of Sr-ions and network of [Sr(H4 AMP)] (top); 3D structure of [Ba(H4 AMP)(H2 O)] (amino-tris(methylenephosphonic) acid = H6 AMP); adapted
from [77].
Onoda et al in the synthesis of the polymer [Ca{O3 POC6 H3 2,6-(NHCOPh)2 }(H2 O)4 (EtOH)], which extends into a onedimensional zig-zag chain, and of a cyclic octameric structure of
[Ca8 {O3 POC6 H3 -2,6-(NHCOPh)2 ]8 (dmf)8 (H2 O)12 ] (Fig. 35)
[78]. The zig-zag structure transformed into a cyclic octanu-
Scheme 9. Coordination of the phosphonate to the metal ions Ca2+ ; by permission of the Royal Society of Chemistry from [78].
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clear structure due to the change of coordination of DMF and
the intermolecular hydrogen-bond network. The ligand design
with amide groups is an intriguing approach for the regulation of
intramolecular and intermolecular hydrogen bonds. The synthesis of Ca2+ complexes using strategically designed bulky amide
ligands will become very important for understanding biomineral Ca2+ structures found in biological systems and also for
expanding new Ca2+ cluster chemistry.
3.2.5. Sulfonates
In terms of anionic oxygen donor ligands to alkaline earth
metal ions in polymeric scaffolds, the sulfonates have been
quite well studied. Thus, a family of organosulfonates has
been reported by Côté et al. [79]. They have developed new
solids, which, in contrast to more typical crystal engineering
approaches, are sustained by the assembly of building blocks that
are coordinatively adaptable rather than rigid in their bonding
preferences. The ligand, 4,5-dihydroxybenzene-1,3-disulfonate,
L, progressively evolves from a 0D, 1D, 2D, to a 3D microporous
network with the Group 2 cations Mg2+ , Ca2+ , Sr2+ , and Ba2+
(Fig. 36). This trend in dimensionality can be explained by considering factors such as hard-soft acid/base principles and cation
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Fig. 35. Transformation of a 1D structure into a ring-type structure upon addition of DMF as ligand [78].
radii, a rationalization, which follows salient crystal engineering
principles.
With H2 S at 50 ◦ C, a mass increase of 7.5% was observed
for the barium compound, corresponding to the uptake of 0.93
equivalents of H2 S with respect to the equivalent of expelled
water. Additional evidence for the sorption of H2 S was obtained
using a gravimetric analysis. A sample of [Ba(L)(H2 O)](H2 S)x
was prepared by mono-dehydrating the initial barium compound
in a N2 purged flask, followed by H2 S purging. After removing excess H2 S, the sample was dissolved in 1 M Pb(NO3 )2 . A
highly insoluble black PbS precipitate formed instantaneously,
indicating H2 S in the sample. Quantification of the precipitate
showed 0.62 equivalent of H2 S was included by [Ba(L)(H2 O)].
A lower measured capacity by this method than by the thermobalance is to be expected due to loss of H2 S during sample
handling. Significantly, no other guests showed any uptake,
implying selectivity for H2 S.
Incomplete deprotonation of tris-sulfonates leads to robust
cationic frameworks with channels between pillaring ligands in
which reside the chloride anions of [Ba2 (L)(H2 O)Cl] (Fig. 37).
This barium compound contracts slightly upon dehydration but
retains its overall structural motif to 420 ◦ C. Significantly, the
chloride ions of the structure can be exchanged in 80% yield for
fluoride ions in a facile manner. This exchange is quantified by
Fig. 36. One-, two- and three-dimensional structures of the calcium, strontium and barium sulfonates, respectively; adapted from [79].
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Fig. 38. 1D chain structure of [Ca(gly)2 ]H2 O; adapted from [82].
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Fig. 37. Structure of [Ba2 (L)(H2 O)Cl] down the a-axis with Cl− in the channels
[80].
elemental analyses, gravimetric determination, and 19 F NMR
spectroscopy. Confirmation of retention of structure is provided
by standardized powder X-ray diffraction experiments. This last
point is notable as the F-analogue of the structure is not attainable
by a direct synthesis. These results illustrate one of the hallmark
features of supramolecular chemistry that a robust and functional
framework can result through cooperative interactions between
more weakly interacting units [80].
3.3. Mixed N- and O-donors
Self-assembly is an increasingly viable method for the quick
and efficient construction of large molecular architectures. Porphyrins are particularly suitable building blocks for non-covalent
synthesis due to their attractive photophysical, spectroscopic,
geometrical, catalytic, and synthetic properties [81]. Furthermore, the magnesium complex of porphyrin is responsible for
photosynthesis. Thus, a mixture of 1:1 magnesium dicatechol
porphyrin MgL and pyridine-3-boronic acid self-assembles into
a one-dimensional coordination polymer (Scheme 10). The
boronic acid acts as N- and O-donor in a bridging function
towards two magnesium cations.
Another group of simultaneous N- and O-donating ligands of biological importance are the amino acids. A
series of calcium ␣-aminocarboxylates was prepared by
refluxing aqueous solutions/suspensions of calcium hydroxide and the respective ␣-amino acid [82]. The colorless,
crystalline hydrates Ca(gly)2 ·H2 O (Fig. 38), Ca(ala)2 ·3H2 O,
Ca(val)2 ·H2 O, Ca(leu)2 ·3H2 O, Ca(met)2 ·nH2 O (n = 2), and
Ca(pro)2 ·H2 O have been isolated in yields between 29
and 67% (gly = glycinate, ala = rac-alaninate, val = rac-valinate,
leu = rac-leucinate, met = rac-methioninate, pro = rac-prolinate).
The compounds are readily soluble in water. The 0.10 M
solutions have a pH of ca. 11, which is consistent with a
noticeable degree of dissociation. The 13 C NMR spectra point
to carboxylate coordination in solution, but no indication of
nitrogen coordination was found. The complete single-crystal
X-ray structure analyses of the first four derivatives reveal
that all aminocarboxylate ligands are N,O-chelating. Crystals
of the alanine derivative consist of mononuclear complexes,
while the other five compounds form three different types of
one-dimensional coordination polymers. Structural diversity is
also observed with the binding modes of the aminocarboxylate ligands and the calcium environment. Besides terminal
aminocarboxylate coordination, there are three different types
of aminocarboxylate bridges. The calcium ions are seven- or
eight-coordinate in N2 O5 and N2 O6 coordination environments,
respectively; one or three water molecules are part of the first
ligand sphere of each metal ion. The crystal structures support
conjectures about the existence of the yet undetected solution species [Cax (aa)2x (H2 O)n ] (aa = ␣-aminocarboxylate). For
example, x = 1 is realized in crystalline [Ca(ala)2 -(H2 O)3 ], and
in [Ca2 (leu)4 (H2 O)4 ] complexes (x = 2) are linked to infinite
chains by bridging aqua ligands.
Sulfonylamines represent yet another type of ligand with
combined O- and N-donor ligands. As for alkali metal ions, their
alkaline earth metal derivatives have been studied and possess
one-dimensional polymeric structures in the case of the com-
Scheme 10. Porphyrin (left), boronic acid (middle) and one-dimensional Mg-polymer (right), adapted from [81].
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Fig. 39. 1D structure of [Ba(BSA)2 (H2 O)2 ] (left) and 2D network of [Ba(2-aba)2 (OH2 )] (right); adapted from [83].
Scheme 11. 2-Aminobenzoic acid with the amino function in the meta position for perfect chelate formation with Mg, Ca, Sr and Ba; adapted from [84].
bination of barium cations with benzene-1,2-di(sulfonyl)amine
(BSA) (Fig. 39) [83].
Combining one carboxylic group with an amino function
within a ligand may lead to additional coordination of the latter to the metal ions, depending on the size of the cations
and the reaction conditions. Thus, 2-aminobenzoic acid has
the amino function placed in the meta position, allowing for
a perfect chelate formation when possible (Scheme 10) [84].
Reactions of alkaline earth metal chlorides with 2-aminobenzoic
acid (2-abaH) in a 1:2 ratio in a MeOH/H2 O/NH3 mixture
leads to the formation of anthranilate complexes [Mg(2-aba)2 ],
[Ca(2-aba)2 (OH2 )3 ], [{Sr(2-aba)2 (OH2 )2 }H2 O)], and [Ba(2aba)2 (OH2 )] (Scheme 11). Alternatively, these products can
also be obtained starting from the corresponding metal acetates.
The solid-state structures of calcium, strontium and barium
metal anthranilates have been established by single-crystal
X-ray diffraction studies (Fig. 40). While the calcium ions
are hepta-coordinated, the strontium and barium ions reveal
a coordination number of 9 apart from an additional weak
metal–metal interaction along the polymeric chains. The carboxylate groups show different chelating and bridging modes
of coordination behavior in the three complexes. Interestingly,
apart from the carboxylate functionality, the amino group also
binds to the metal centers in the case of strontium and barium complexes. However, the coordination sphere of calcium
in its compound contains only O donors. All three compounds form polymeric networks in the solid state with the
aid of different coordinating capabilities of the carboxylate
anions and O–HO and N–HO hydrogen bonding interactions.
As mentioned earlier, the calcium coordination polymer with
2-aminobenzoic acid features only oxygen donation towards the
cation [84]. The barium derivative [Ba(2-aba)2 (OH2 )] however,
has the ligands in two different coordination modes, one of which
is based on solely O-donation, while the second ligand molecule
also involved the N-atom in metal ion coordination.
Investigation of the effect of pH in carboxylic acid systems
is of particular importance because it helps us to understand the
correlation between the reaction acidity and the structure dimensionality. Combining two carboxylic groups with N-donors, as
in 3,5-pyrazoledicarboxylic acid (H3 pdc), should led to diverging coordination of the ligand and thus to polymeric structures
as a function of the pH, because carboxylic groups can be deprotonated, N-atoms additionally protonated. The acidity levels of
the reactions can be adjusted not only by adding basic solvents
but also by introducing basic precursors, such as Ca(OH)2 ,
Fig. 40. The Sr-compound [{Sr(2-aba)2 (OH2 )2 }H2 O)]; adapted from [84].
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Fig. 41. Crystal structures of the Ca-, Ba- and Sr-structures obtained with 3,5-pyrazoledicarboxylic acid as coordinating bridging ligand; adapted from [85].
Sr(OH)2 , and Ba(OH)2 . Indeed, several alkaline earth metal
coordination polymers were obtained as a function of pH and
thus deprotonation grade of the ligand [85]. At a lower pH level
three one-dimensional structures [Ca(Hpdc)(H2 O)4 ]·2H2 O,
[Ca(Hpdc)(H2 O)4 ]·H2 O, which are pseudo-polymorphs (or
solvates), and [Ba(H2 pdc)2 (H2 O)4 ]·2H2 O were obtained by
evaporation of the solutions resulting from hydro(solvo)thermal
reactions of MCl2 (M = Ca, Ba) with H3 pdc in water or in
water/Et3 N at 150 ◦ C for 3 days. The first two compounds are
based on zig-zag chains of metal ions bridged by a single Hpdc2−
ligand, whereas the barium structure consists of linear chains of
metal centers bridged by two H2 pdc− ligands. At higher pH levels (pH 4–6), the three-dimensional polymers [M(Hpdc)(H2 O)]
(Ca, Sr, Ba) were isolated by reactions of MCl2 (M = Ca, Sr, Ba)
with H3 pdc in water/Et3 N or in M(OH)2 (M = Ca, Sr, Ba) with
H3 pdc in water under hydro(solvo)thermal conditions (150 ◦ C,
3 days). Calcium and strontium are seven- and nine-coordinated
in their respective compounds, while barium is nine- and
ten-coordinated (Fig. 41). It was observed that the increase in
pH resulted in a higher connectivity level of ligands, which in
turn leads to a higher dimensionality of the crystal structures.
At lower pH levels, alkaline earth metal elements show a high
tendency to coordinate with water molecules, resulting in a high
water-to-metal ratio, thus limiting their ability to form structures
of higher dimensionality as rare-earth metals and cadmium. The
comparisons among different metal–pdc species also show that
the hydrogen atom attached to the pyrazole ring of the dicarboxylate cannot be deprotonated in either rare-earth or alkaline-earth
metal compounds.
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3.4. N-donor ligands
Neutral N-donor ligands are, just as neutral O-donors, able
to stabilize low-dimensional coordination polymers of alkaline
earth metal salts. The polymer structures are generated in most
cases by bridging halides, while the N-donor molecules step in as
terminal ligands to reduce dimensionality of the initial polymer
structure [86].
Even though that 4,4 -bipyridine is a widely used ligand
in transition metal ion coordination, examples with alkali and
alkaline earth metal ions are rather scarce due to the general
preference of block s elements towards oxygen. Complete elimination of oxygen donors leads however to alkaline earth metal
compounds, in which the metal ions are entirely coordinated
by N-atoms. This is especially true for the larger metal ions,
as they have a lower charge density and are softer than their
smaller homologues. However, steric bulk of the ligand needs
to be sufficient to stabilize the coordination.
Monoanionic heteroallylic ligand systems [R–N–E–N–R]−
(E = Si(R2 ), S(R2 ) or S(R), C(R), and P(R2 )) are versatile
chelating substituents both in main group and transition metal
chemistry as they provide sufficient steric demand and solubility to the products [87]. Their application is only limited by the
rigid bite of the ligands as the N–N distance cannot be tuned to
the various radii of different metals. The NP(R2 )N− chelate in
classical aminoiminophosphoranates is extended by additional
coordination sites in the organic substituents (e.g. , 2-pyridyl
(Py) instead of phenyl (Ph)). Py2 P{N(H)SiMe3 }(NSiMe3 ) is
the starting material for a new class of complexes as the
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While the 4,4 -bipyridine is a linear directing ligand,
tetrakis(imidayolyl)borate points into the four corners of a
tetrahedron [88]. With magnesium, calcium and strontium,
the compounds of general composition M[B(Im)4 ]2 (H2 O)2
(M = Mg, Ca, Sr) are obtained with the same coordination
environment about the metal ions. However, the three compounds have different network structures with different degrees
of hydrogen bonding; the Mg material forms a two-dimensional
network and the Ca and Sr compounds form one-dimensional
chains (Fig. 43).
The formation of interlaced squares in Mg[B(Im)4 ]2 (H2 O)2
versus one-dimensional chains in the Ca[B(Im)4 ]2 (H2 O)2 and
Sr[B(Im)4 ]2 (H2 O)2 structures results from how the borates can
link together metals in either a one- or two-dimensional network.
As can be seen in the three metal–organic networks presented
in this report, the nature of the metal ion can be used to tune the
structure of the solid. In the cases presented here, a combination of the radius of the metal and its electronic character affect
the topology of the resultant solid. Several parameters change
as one goes down the group, including the closed/open conformation of the borate, the geometry of the imidazoles about the
metal center, and the lengths of the metal–ligand bonds. These
variations in geometries and hydrogen bonding result in differences between the magnesium structure, which adopts a brick
wall layered pattern, and the calcium and strontium structures,
which form one-dimensional square chains.
3.5. Carbon donor ligands
Fig. 42. 4,4 -Bipyridine as bridging ligand for the formation of a 1D-polymer
of [(4,4 -bipy)Ba{Py2 P(NSiMe3 )2 }2 ]; adapted from [87].
deprotonated ligand contains along with the NPN-chelate
the pyridyl ring nitrogen atoms to generate a side selective
Janus face ligand. In [(THF)Sr{Py2 P(NSiMe3 )2 }2 ] and [(4,4 bipy)Ba{Py2 P(NSiMe3 )2 }2 ] both pyridyl rings are involved in
metal coordination but only one imido nitrogen atom. Hence,
the classical NP(Ph2 )N− chelating ligand is converted into a
NP(Py2 )N− tripodal ligand. Whereas the strontium compound
is a zero-dimensional compound due to its smaller radius and terminal THF-coordination, the bridging bipyridine ligand in the
barium compound allows the formation of a one-dimensional
coordination polymer (Fig. 42).
As discussed for the alkali metal ions, the larger alkaline
earth metal ions also have an increased tendency to interact
with aromatic ␲-systems when available. A general reaction
pathway to such metal–organic compounds is the use of the
alkali-organic compound and its reaction with alkaline earth
metal halides under elimination of alkali halide in an organic
solvent. Thus, indenyl (Ind) and its derivative, bisisopropylindenyl (Ind2i) react as potassium salts with alkaline earth metal
iodides to yield [(Ind)2 M(thf)n ] (M = Ca, Sr, Ba; n = 1 for Sr
and Ba; n = 2 for Ca) and [(Ind2i)2 M(thf)n ] (n = 1 for Ca; n = 2
for Sr and Ba) under KI elimination [89]. All complexes are
air-sensitive solids, and while the calcium and barium compounds are monomeric, the strontium complex is an infinite
coordination polymer, [(Ind)2 Sr(thf)], with both terminal (Sr–C)
2.94 Å (av)) and bridging (Sr–C) 3.07 Å (av)) indenyl ligands
(Fig. 44).
Fig. 43. Different networks obtained with Ca and Sr (left, a, and right), and Mg (b) and tetrakis(imidayolyl)borate; adapted from [88].
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Fig. 44. 1D structure of [(Ind)2 Sr(thf)]; adapted from [89].
Such a trans-metallation, using alkali reagents and alkaline earth metal halides in order to eliminate alkali iodide
seems however not always successful [90]. Thus, the
generation of [1,2,4-tris(trimethylsilyl)-cyclopentadienyl]metal
iodides (Cp3 Si)MI(thf)n (M = Ca, n = 1; M = Sr, Ba, n = 2) are
isolated from the 1:1 reaction of K[Cp3 Si] and MI2 in THF,
but the yields range from very good (79%) when M = Ca to
poor (26%) when M = Ba. In the case of M = Sr, Ba, substantial
amounts of K[Cp3 Si] are recoverable at the end of the reaction. No redistribution of (Cp3 Si)MI(thf)n into (Cp3 Si)2 M and
MI2 (thf)n is observed in either THF or aromatic solvents at
room temperature. Both (Cp3 Si)CaI(thf) and (Cp3 Si)SrI(thf)2
crystallize from THF/toluene as iodide-bridged dimers, while
the organobarium complex (Cp3 Si)BaI(thf)2 crystallizes from
THF/toluene as a coordination polymer containing both linear
and near-linear (177.8◦ ) Ba–I–Ba links in a zig-zag motif; this
is an unprecedented arrangement for bridging iodide ligands
(Fig. 45).
3.6. Sulfur donor ligands
Combination of carboxylate groups with sulfur donor
groups such as thiols, additional coordination of sulfur to the
cations may be observed in the case of the correct design of
the ligand. Treating alkaline earth metal chlorides with the
ligand 2-mercaptobenzoic acid (Scheme 12), extended polymer structures [Ca(DTBB)(H2 O)2 ·0.5(C2 H5 OH)], [Sr(DTBB)
(H2 O)2 ·0.5(C2 H5 OH)], and [Ba2 (DTBB)2 (H2 O)2 ·0.5H2 O] are
obtained, in which the ligand underwent thiol oxidation to form
2,2 -dithiobis(benzoic acid) (H2 DTBB) as a bridging ligand
[91]. The calcium and strontium coordination polymers are isomorphous with the DTBB ligands acting as hexadentate ligands.
Both compounds have a channel structure in which solvent
ethanol molecules are included. In the barium compound, a complex three-dimensional coordination polymer where both the
carboxylate and the sulfur groups of the DTBB ligands (which
are hepta- and octadentate) coordinate to the metal, is observed
(Fig. 46).
While the coordination is exclusively through the carboxylate
groups of the DTBB ligands in the case of hard metal ions such
as calcium and strontium, the disulfide linkage also participates
in the metal binding in the case of the softer barium ion. The
binding of disulfide linkage to barium ion may have some role to
play in heavy metal poisoning by barium in physiological systems. This observation probably needs to be further investigated
in future studies.
Fig. 45. 1D-coordination polymer of (Cp3 Si)BaI(thf)2 (Asymmetric unit left, polymer arrangement right); adapted from [90].
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Scheme 12. Reaction of 2-mercaptobenzoic acid with alkaline earth metal chlorides; adapted from [91].
Fig. 46. 2-Mercaptobenzoic acid as O-donor ligand to Ca (top) and as O- and S-donor to Ba; adapted from [91].
4. Conclusions
marised here, but which are also transferable to other ligand
systems:
4.1. Alkali metal coordination polymer networks
The construction of different dimensional coordination polymer networks based on alkali or alkaline earth metal ions is
mainly based on anionic oxygen donor ligands. Even though
a number of results have been found in the literature, most
results seem to be based on serendipitous discoveries rather than
systematic constructions.
The synthesis of alkali metal coordination polymers can be
achieved using a salt and forming low-dimensional structures
with neutral donor ligands, the latter acting as chemical scissors
on the initial material. With anionic ligands, several synthetic
methods to access polymeric structures with alkali metals are
possible. Examples for the formation of alkoxides are sum-
Reactions of metals and alcohols
Reactions of metals hydrides
with alcohols
Metal–carbon bond cleavage
reactions
Reactions of metal dialkyl
amides with alcohols
M + ROH → MOR + (1/2)H2
MH + ROH → MOR + H2
nBuLi + ROH → LiOR + nBuH
M{N(SiMe3 )2 } + ROH → MOR
+ HN(SiMe3 )2
The interest in these reactions clearly relies on the fact that
the secondary product is volatile, allowing easy purification of
the product.
Reactions of metal hydroxide with alcohols :
MOH + ROH → MOR + H2 O
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The water formed during this reaction has to be eliminated
by azeotropic distillation, for instance with NaOH dissolved in a
mixture ethanol/benzene under reflux. The more acidic phenols
allow formation of most group 1 phenoxides more readily by
simple heating of the hydroxide in dried ethanol, followed by
recrystallization.
For the smaller and thus harder alkali cations, lithium and
sodium, the cations are coordinated by the donor atoms of the ligand, and the dimensionality of the resulting solid-state structure
can be governed by the capacity of the ligand to bridge between
the metal ions, i.e. its steric demand as well as its number and
position of donor sites.
Potassium aryloxides KOR, as well as the heavier analogues
rubidium and cesium, often possess polymeric structures due to
the large ionic radius and the low charge of the cation, giving
them a relatively soft character. In general, such adducts with
less sterically demanding ligands present solvated polymeric
structures, while by increasing the steric bulk, unsolvated polymers may be observed. Usually, the large cations are then also
␲-bonded to the aryl rings of the neighboring ligand to fill their
unsaturated coordination sites.
Summarising the situation for alkali metal ions containing
coordination polymer structures, even though a large number of
materials are used as their group 1 metal salts, the structures
of the latter are often not investigated in detail, ignoring the
cations as simple “spectator” ions. However, this is an issue
worth investigating in order to better understand the properties
of these solid-state compounds. Few of the examples mentioned
above have been investigated in this respect, and first recent
results show that a certain design in the construction of alkali
metal coordination polymer structures can be achieved by a)
using (rigid) bridging ligands with two or more coordination
sites, and/or b) template molecules to generate voids in the solidstate structures, and/or c) the use of alkali metal aggregates as
nodes of a low-dimensional network to yield porous materials
with large cavities.
4.2. Alkaline earth metal coordination polymer networks
The alkaline earth metal ions have received more attention,
probably due to the research on group 2 metal containing precursors for oxide as well as biomimetic materials. In terms of
networks and nodes, moving from group 1 to 2 metals halves
the number of nodes and so decreases connectivity for the same
ligand types.
Again by serendipity, many polymeric species have been
discovered, but also more systematic studies have been made
including a series of group 2 cations and comparing the structures obtained. There is however no clear trend in the evolution of
structural features with respect to the metal ion size. One expects
that the larger the cation, the higher its coordination number
and thus the structure will display a higher dimension. However, some of the above examples show isostructural compounds
for calcium, strontium and barium despite the softer character
of barium compared to calcium. Some trends are still applicable, as for the alkali metal ions. Thus, the larger the cation, the
lower its charge density, and the higher its tendency to com-
28
plete its coordination sphere by ␲-interactions with aromatic
ligands.
With neutral O- and N-donor ligands, several lowdimensional networks were isolated and characterized, using
the ligands as chemical scissors. The generation of anionic ligands can be summarised in the following methods, exemplified
by carboxylates:
• Reaction of alkali carboxylate with group 2 halide:
2MOOR + M X2 → M (OOR)2 + 2MX
• Reaction of carboxylic acid with group 2 hydroxide:
2HOOR + M (OH)2 → M (OOR)2 + 2H2 O
The latter reaction can be fine-tuned as a function of the pH, as
the systematically described studies showed. This is especially
interesting in the case of ligands with more than one functional
group to be deprotonated. Then, the bridging functionality of
the ligands can be switched on or off as a function of the pH,
and thus the dimensionality of a coordination polymer network
can be tuned.
Highlights in the generation of coordination polymer networks in terms of functionality are the rigid dicarboxylate
derivatives with Mg3 -units as nodes that show a porosity and
thus host-guest behaviour in the solid state, the use of amino
acids and their derivatives as ligands on group 2 ions for a better
understanding of biological systems, and the alkoxides which
are able to react with CO2 and SO2 to yield new products.
For both, alkali and alkaline earth metal MOFs, the presence of water molecules in the coordination sphere of the metal
ions strongly influences the final structures. Water molecules are
highly attracted to s-block ions, and are small enough to coordinate easily to any of the metal ions. This is due to the fact that
groups 1 and 2 metal ions usually are able to rapidly exchange
their neutral ligands in solution, and that, if water is present, the
latter may then often bind to the metal ions in a stronger fashion
than another neutral ligand.
All in all, the formation of metal–organic frameworks
(MOFs) with alkali and alkaline earth metal ions has not been
exhaustively investigated. Whereas mostly solution synthesis
or autoclave techniques for groups 1 and 2 coordination polymers are employed, microwave and solid-state synthesis are still
scarce for these kinds of networks. Additionally, applications
in many, very different fields are possible, and therefore such
investigations should be continued.
Acknowledgements
The author thanks the Swiss National Science Foundation for
generous funding.
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